All four of the valves in the heart are passive structures in that they do not themselves expend any energy and do not perform any active contractile function. They consist of moveable “leaflets” that open and close in response to differential pressures on either side of the valve. The problems that can develop with valves can generally be classified into two categories: (1) stenosis, in which a valve does not open properly, and (2) insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload placed on the heart. The severity of this increased stress on the heart and the patient, and the heart's ability to adapt to it, determine the treatment options that can be pursued. In some cases, medication can be sufficient to treat the patient, which is the preferred alternative; however, in many cases defective valves have to be repaired or completely replaced in order for the patient to live a normal life.
The two general categories of valves that are available for implantation into the cardiac system are mechanical valves and bioprosthetic or tissue valves. Mechanical valves have been used for many years and encompass a wide variety of designs that accommodate the blood flow requirements of the particular location where they will be implanted. Although the materials and design features of these valves are continuously being improved, they do increase the risk of clotting in the blood stream, which can lead to a heart attack or stroke. Thus, mechanical valve recipients must take anti-coagulant drugs for life to lessen the potential for blood clot formation. Further, mechanical valves can sometimes suffer from structural problems that may force the patient to have additional surgeries for further valve replacement.
Bioprosthetic valves, which are also referred to as prosthetic valves, generally include both human tissue valves and animal tissue valves. The designs of these bioprosthetic valves are typically relatively similar to the design of the natural valves of the patient and advantageously do not require the use of long-term anti-coagulant drugs. Human tissue valves are typically not available in large quantities, however, since they must be removed from deceased persons who have elected organ donation. On the other hand, due to the large numbers of animals routinely processed at meat processing facilities, for example, animal tissue valves are more widely available for the patients who require valve replacement. The most common types of animal tissue valves used include porcine aortic valves, and bovine and porcine pericardial valves, some of which are incorporated with some type of a stent before implantation in a patient. In the case of pericardial valves, the use of pericardial material to design and make the heart valves provides a much larger range of design options than is available when using only harvested valves.
In order to incorporate a tissue valve with a stent or other type of frame, a number of different techniques and methods have been used, such as clamping, tying, gluing, or stitching, for example. However, many of the techniques used for this purpose generally produce a stented valve that has concentrated stresses at the points where the leaflets are attached to the stent frame. That is, because the stents are relatively rigid as compared to the flexible material from which the leaflets of the tissue valve are made, the repetitive flexing motion of the leaflets can create stress concentrations at the points where the tissue valve is attached to the stent. These stress concentrations can eventually lead to tearing of the tissue, valve leakage, and/or failure of the heart valve. The attachment points can also be sites for abrasion of the tissue that can lead to tearing of the tissue. Thus, there is a continued need to be able to provide methods and devices for a durable attachment between a tissue valve and a stent and/or to distribute the stresses away from the attachment and seam areas and provide for nonabrasive contact surfaces for bioprosthetic heart valve leaflets.